This invention relates to a positron source.
It has a very large number of applications, particularly in solid state physics, in material sciences and in surface physics, in which a high counting rate is important for many applications, for example such as a scanning positron microscope, lifetime measurements as a function of the implantation depth or Doppler broadening, and PAES (Positron annihilation induced Auger Electron Spectroscopy).
Other applications of the invention use positronium xe2x80x9catomsxe2x80x9d directly (positronium being the bound state of an electron and a positron). However, the production of positronium also requires a large number of positrons.
The invention is also applicable in molecular chemistry and more particularly to the determination of processes involved in superconducting materials with high critical temperature.
It is equally applicable to determination of the aging capacity of paints and coatings.
Furthermore, the invention is also applicable to the detection of defects in a material, as it is known that annihilation of positrons is sensitive to the electron density. For example, small variations of this density are detected when the material thermally expands. Vacancies, in other words single atoms missing from the lattice of a crystalline material, are then very easily detected due to their low electron density. Concentrations of vacant atomic sites of the order of 10xe2x88x926 (1 ppm) are observable.
Since a material is analyzed by a contact free positron beam, the material can be heated to a very high temperature. Vacant sites may also be introduced at any temperature by mechanical deformation, sputtering or ion implantation.
The adjustable energy of the positron beam is a means of obtaining in-depth information with a resolution of 10% for structures in thin layers or samples comprising a non-uniform distribution of defects.
Furthermore, electric fields in oxides of microelectronic devices such as MOSs can be used to deviate positrons at the study interface.
Vacancy clusters forming cavities of the order of 0.5 nm can easily be observed by variations of Doppler broadening and the lifetime of positrons.
Observing the formation of positronium demonstrates the presence of broader cavities and can determine their size (up to 20 nm).
For even larger cavities, ortho-positronium (state of the positronium in which electron and positron spins are anti-parallel) survives long enough for it to disintegrate into three photons. In this case, the angular correlation of photons gives an increase on Doppler broadening by a factor of 5.
Note also other applications of the invention:
PRS (Positron Re-emission Spectroscopy),
PAES (Positron annihilation induced Auger Electron Spectroscopy),
REPELS (Re-Emitted Positron Energy Loss Spectroscopy),
LEPD (Low-Energy Positron Diffraction),
PIIDS (Positron Induced Ion Desorption Spectroscopy),
PALS (Positron Annihilation Lifetime Spectroscopy), this technique being extremely important in microelectronics,
VEPLS (Variable Energy Positron Lifetime Spectroscopy), and
PAS (Positron Annihilation Spectroscopy).
This invention relates more particularly to production of a low energy positron beam, less than 10 MeV, with an instantaneous intensity of more than 1010 positrons per second, and preferably more than 1012 positrons per second, for example in order to obtain:
a low energy positron beam, with an energy of less than 10 kev by coupling with an appropriate trap, or
positronium atoms, by interaction with an appropriate target.
Production with a high rate (more than 1010 per second), of low energy positrons and positronium xe2x80x9catomsxe2x80x9d is necessary for industrial applications such as measuring defects in crystals or organic materials, when for example PAS (Positron Annihilation Spectroscopy) or other methods mentioned above are used.
These applications use mainly 22Na sources as positron beam sources. These compact sources are very suitable for laboratory research. But their maximum activity is about 4xc3x97109 Bq and their average lifetime is only 2.6 years.
Moreover, there are some accelerators for which part of the activity, frequently minor, relates to production of positron beams. However, these are mainly large and expensive installations since the energy of electrons used is very frequently several tens of MeV, typically 100 MeV. Positrons emitted may have energies of several tens of MeV.
Moreover, positrons useful for industrial applications have a kinetic energy less than at least one thousand times the energy of the production threshold. Conventionally, metallic moderators with very low efficiency (less than 0.001) are used to slow them.
Furthermore, it is known how to trap a positron beam in a device called a Penning-Malmberg trap. An improved trap, called the Greaves-Surko trap, enormously increases the brightness of the beam by dividing the energy dispersion of this beam by a thousand, with an efficiency of the order of 1.
Greaves-Surko traps are commercially available from the First Point Scientific Company. They comprise a solid neon moderator whose efficiency is close to 1%.
These traps are very advantageous for the above-mentioned applications and since their appearance they have become more widely used, but the energy of the positrons must be less than 1 MeV.
Furthermore, four techniques are known for producing positrons. These techniques use radioactive sources (of 22Na type) or neutron fluxes from nuclear reactors or tandem accelerators (designed to accelerate ions) or electron accelerators.
We will now examine the disadvantages of these techniques.
The positron current output by a radioactive source is limited by the thickness of the material surrounding the source. Furthermore, the intensity of the positron beam emitted by such a source is of the order of 108 e+/s and therefore of the order of 106 e+/s after moderation.
The use of neutron fluxes output from a nuclear reactor provides a means of obtaining short lifetime radioactive sources capable of producing low energy positrons. However, this technique cannot be industrialized because it requires a nuclear reactor.
One variant of the previous technique consists of using a tandem accelerator to accelerate ions that are sent to a target. This target becomes radioactive and emits low energy positrons. Although a tandem accelerator is smaller than a conventional particle accelerator, it forms a large and expensive installation that requires protection against activation and a maintenance infrastructure.
Large linear accelerators, more simply called xe2x80x9clinacsxe2x80x9d, are also used to produce positrons, by accelerating electrons and sending them to a tungsten or tantalum target. However, these large linacs are very large and expensive installations and there are not enough of them to facilitate the development of positron applications of the type mentioned above.
Let us reconsider known interaction chambers containing a target that is capable of generating positrons by interaction with an electron beam.
To produce positrons (denoted e+) from an electron beam (denoted exe2x88x92), these electrons have to interact with a target material. The electrons then emit X and gamma photons which sometimes disintegrate in pairs (e+ exe2x88x92).
Since the number of positrons produced depends on the number of electrons that interacted with the target material, a person skilled in the art will decide to use intense beams like those produced by linac type accelerators.
Since the number of e+ produced by an electron beam increases with the thickness of the target passed through, a person skilled in the art would tend to increase this thickness.
But two problems then arise.
Firstly, the X rays deposit energy in the form of heat in the target.
Secondly, the e+ created can be captured in the target and annihilate before exiting from the target. This annihilation may take place according to two reactions, namely direct collision with an electron or the formation of a positronium atom.
A person skilled in the art naturally associates the use of a thick target with the use of high energy accelerators.
Systems that produce high energy e+ (more than 10 MeV) for particle physics experiments are not as sensitive to the second problem because high energy e+ do not annihilate, and particularly do not form positronium. Furthermore, for industrial applications in which e+ must have a very low energy, the formation of positronium along the path between the location at which the e+ is created from the target exit point destroys a large proportion of the e+.
On the other hand, the first problem becomes very penalizing at high energies. For a given amount of heat deposited in the target, a 100 MeV electron beam generator and a 10 MeV electron beam generator provide the same number of xe2x80x9cusefulxe2x80x9d positrons with an energy of less than or equal to 1 MeV.
For example, consider firstly the existing technique for a 100 MeV generator sending electrons to a 1 mmxc3x971 cm2 target at 90 degrees, and secondly a 10 MeV generator sending electrons to a 50 xcexcmxc3x971 cm2 target at 3 degrees as proposed according to one example of this invention. For the same heat deposited in the target, and a similar number of useful e+ produced, the 100 MeV generator will consume 50 kW and a 10 MeV generator will consume 10 kW. The 40 kw difference is wasted and must be evacuated from the collection system in the form of heat.
In order to use a larger proportion of the positrons produced, the large installations using a high energy linac such as the Lawrence Livermore National Laboratory in California (USA), and the ISA (Institute for Ring Storage Facilities, University of Aarhus (Denmark)), use tungsten deceleration sheets placed behind the target, possibly combined with an appropriate electric field. But this type of device absorbs many positrons, in other words it limits the beam intensity.
The purpose of this invention is to overcome the disadvantages mentioned above.
Its purpose is a positron source, this source comprising means of generating an electron beam and a target that comprises a substantially plane surface, this target being designed to receive an electron beam on this substantially plane surface, at a predetermined angle of incidence, counted with respect to the substantially plane surface, and to generate positrons by interaction with this electron beam, this source being characterized in that the generated electron beam is continuous or quasi-continuous and the energy of the electrons is of the order of 10 MeV, and the target thickness is less than 500 xcexcm and the predetermined angle of incidence is less than 10xc2x0.
According to one preferred embodiment of the positron source according to the invention, the thickness of the target is within the interval ranging from 10 xcexcm to 100 xcexcm and the predetermined angle of incidence is within the interval ranging from 2xc2x0 to 5xc2x0.
Preferably, the electron beam generation means generate a continuous beam and comprise an electron accelerator comprising a coaxial cavity that electrons pass through several times in a median plane perpendicular to the axis of this cavity.
This electron accelerator is known under the term xe2x80x9cRhodotronxe2x80x9d (registered trademark) and is described in the following document:
FR 2616032 A corresponding to U.S. Pat. No. 5,107,221 A.
In one preferred embodiment, this invention also comprises sorting means between positrons and electrons that did not interact with the target, said sorting means comprising:
first magnetic means, whose axis is close to the beam axis and passes through the plane of the target, and which are designed to generate a magnetic field that can make positrons emitted by the target diverge, these first magnetic means being arranged on the input side of the target at an appropriate distance,
a magnetic quadrupole for focusing the positron beam, said magnetic quadrupole having the same axis as the first magnetic means, being placed on the output side of the target, and being designed to make the positron beam section circular, said positron beam being very flat at the output from the area of the interaction between electrons and the target,
first stop means, located on the axis of the first magnetic means, on the output side of the quadrupole, at a sufficiently long distance to focus positrons into a beam with a circular section, said first stop means being designed to stop electrons from the electron beam that did not interact with the target,
second magnetic means, along the same axis as the first magnetic means, arranged on the output side of the first stop means, at an appropriate distance from the first magnetic means to generate a magnetic field capable of making the positrons converge, the first and second means cooperating to generate a magnetic field capable of preventing these positrons from encountering the first stop means.
The positron source according to the invention may also include trapping means, designed to trap positrons generated by the target.
The trapping means comprise a moderator designed to decelerate positrons and electromagnetic means of concentrating these positrons.
These trapping means may comprise a Greaves-Surko trap, about which one may refer to the following document:
R. Greaves and C. M. Surko, Nucl. Inst. Meth. B192 (2002) 90.
Preferably, the positron source according to the invention also comprises:
second stop means, for example a lead wall cooled by water circulation, designed to stop electrons in the electron beam which did not interact with the target and which reached a zone between the second magnetic means and the trapping means, and to prevent these electrons from reaching the trapping means, and,
means of guiding positrons towards the trapping means through these second stop means.